Thanks a lot. Almost lost a game because of that argument. Yeah and people tend to forget that Beginning of Combat is different phase than Declare Attacker's step.
The Beginning of Combat Step and the Declare Attackers Step are not in different phases. They are in the same phase. As such, if you play Hurkyl's Recall during the Beginning of Combat step, your opponent can add mana before it resolves and will still have the mana in his pool during the Declare Attackers Step to use it to pay for Propoganda.
If you bounce the artifacts during his Main Phase, he will not be able to float mana into his Combat Phase but he will get priority again during his main phase in which he may play said artifacts again.